1. Field of the Invention
The present invention relates to a D/A converter, and more particularly to a distortion reduction circuit which compensates or corrects an error in the D/A converter.
2. Description of the Prior Art
A high precision D/A converter is used in a compact disc player of high performance. When 16-bit digital data is applied to the D/A converter and the preciseness corresponding to one half of the minimum current source is required to be kept over the whole of the bits, an error in the MSB (Most Significant Bit) of the data must be less than 1/65.536.times.1/2.times.100=0.00075%. A current source for an actual D/A converter comprises a standard voltage source, a resistance network, and semiconductor device switches. Since the current value is determined by the resistance value, an error in the resistance value must be less than 0.00075% to maintain an error in the MSB of the input data within.+-.1/2 of the LSB (Least Significant Bit) of the data. It, however, is very difficult to maintain this precision even if laser trimming on the resistance network may be used. Accordingly, a technique to compensate an error in the resistance value by means of external correction circuit has been adopted. One prior art example of such external correction circuit as a distortion reduction circuit for a D/A converter is illustrated in FIG. 1 and also disclosed in Radio Technology by S. Watanabe et al., November, 1985, p.p. 80-84.
In a distortion reduction circuit for a D/A converter 2 shown in FIG. 1, an input line for any bit of parallel input data signal applied to a data, input terminal of the converter 2 branches into two routes .circle.1 and .circle.2 . One route .circle.1 is connected to a terminal "a" of a switch S via a transistor inverter Q1 while the other route .circle.2 is connected to a terminal "c" of the switch S. The switch S is switched in accordnace with the magnitude of deviation, from an ideal value, of a weighted quantity for any particular bit of the input data to output a selected one from the common terminal "b". The selected signal from the terminal b is adjusted by a variable resistor VR1 to an optimal value, and then added to the analog output from the D/A converter 2 at one end of an operational amplifier 3.
In conventional distortion reduction circuits, of the type mentioned above, for D/A converters, however, the polarities of errors in the output levels of the converters corresponding to the detected bits vary depending upon their corresponding converters, so that their switches S must be switched in advance in accordance with their polarities of errors in the output levels from the D/A converters, which is troublesome from the standpoint of operation.
A rise in each input data bit is a rise in the distortion correction signal, so that the actual analog output may not be synchronized with a corrected signal output in which case improvement to distortion in a high frequency area will be reduced.